Apparatus and method for synchronizing power circuits with coherent rf signals to form a steered composite rf signal

ABSTRACT

An apparatus has a Radio Frequency (RF) signal generator to produce RF signals phase shifted relative to one another in accordance with RF frequency waveform parameters. Amplifier chains process the RF signals to produce channels of amplified RF signals. Each amplifier chain has amplifiers and at least one amplifier has a tunable gate voltage synchronized with the RF signals.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims priority to U.S. Provisional Patent ApplicationSer. No. 62/817,096, filed Mar. 12, 2019, the contents of which areincorporated herein by reference.

FIELD OF THE INVENTION

This invention relates generally to high-power Radio Frequency (RF)signal processing. More particularly, this invention is directed towardtechniques for synchronizing power circuits with coherent RF signals toform a steered composite RF signal in a far field.

BACKGROUND OF THE INVENTION

The production of high-power RF signals, such as Megawatts of radiatedpower, typically requires analog RF signal processing circuitry thatconsumes large amounts of energy, which results in large amounts ofradiated heat. Consequently, expensively rated circuits and elaboratecooling mechanisms are typically required in such systems.

Thus, there is a need to produce high-power RF signals with very lowaverage power, such as under five Killowatts.

SUMMARY OF THE INVENTION

An apparatus has a Radio Frequency (RF) signal generator to produce RFsignals phase shifted relative to one another in accordance with RFfrequency waveform parameters. Amplifier chains process the RF signalsto produce channels of amplified RF signals. Each amplifier chain hasamplifiers and at least one amplifier has a tunable gate voltagesynchronized with the RF signals.

BRIEF DESCRIPTION OF THE FIGURES

The invention is more fully appreciated in connection with the followingdetailed description taken in conjunction with the accompanyingdrawings, in which:

FIG. 1 illustrates a system configured in accordance with an embodimentof the invention.

FIG. 2 more fully characterizes components of FIG. 1.

FIG. 3 illustrates a power sequencer utilized in accordance with anembodiment of the invention.

FIG. 4 illustrates power electronics utilized in accordance with anembodiment of the invention.

FIG. 5 illustrates power electronics control signals utilized inaccordance with an embodiment of the invention.

FIG. 6 illustrates an RF signal utilized in accordance with anembodiment of the invention.

FIG. 7 illustrates the system of FIG. 1 utilizing a reflector andmechanical gimbal 702.

Like reference numerals refer to corresponding parts throughout theseveral views of the drawings.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 illustrates an RF signal generating apparatus 100. The RF signalmay be generated in response to a user command entered at a keyboard. Inone embodiment, the RF signal is generated in response to theidentification of a target by a target detector 101, such as a camerautilizing computer vision algorithms. Consider the case of an unmannedaerial vehicle or drone, the target detector 101 collects a signaturecharacterizing the flight attributes of the drone. The target detector101 also collects free space parameters associated with the drone, suchas azimuth angle, elevation and range. Embodiments of the inventioncollect this information when the target is 500 to 300 meters from thetarget detector 101. The signature and free space parameters are passedfrom the target detector to a central computer 102.

The central computer 102 classifies the target and selects RF waveformparameters, which are passed to an RF signal generator 103. The RFsignal generator 103 creates RF signals in accordance with the RFwaveform parameters. Each RF signal has a waveform of the frequency,pulse width, pulse repetition interval and intra-pulse modulationspecified by the RF waveform parameters received from the centralcomputer 102.

The RF signal generator 103 produces RF signals for multiple channelsthat are applied to amplifier chains 104_1 through 104_N. The RF signalsfor the multiple channels are phase shifted relative to one another inaccordance with RF frequency waveform parameters. In one embodiment, thephase shifting is digitally performed within the RF signal generator103. Alternately, analog phase shifters may shift the RF signals priorto applying them to the amplifier chains 104_1 through 104_N.

Each amplifier chain has a serial sequence of solid state poweramplifiers, each of which has a gate voltage on set point derived froman automatic calibration operation, as detailed below. Each amplifierchain produces an amplified RF signal. In one embodiment, a mW RF signalfrom the RF signal generator 103 is amplified to kWs. The amplifierchain may utilize a combination of solid state amplifiers, includingsilicon laterally-diffused metal-oxide semiconductors, Gallium Nitride,Scandium Aluminum Nitride, GaAs and InP.

The channels of RF signals from the amplifier chains 104_1 through 104_Nare applied to an antenna array 106. Each amplifier chain has acorresponding antenna in the antenna array 106. The antenna array 106broadcasts the channels of RF signals as a steered composite RF signalwith Megawatts of radiated power. That is, individual RF signals emittedfrom different antennae in the antenna array 106 interact in free spaceto generate a composite RF signal that is directed to a specifiedlocation corresponding to the location of the target. The antenna array106 may include a mechanical gimbal to position individual antennae.

The RF signal generator 103 also sends control signals to the powersequencer 105. The control signals gate amplifiers in the amplifierchains 104_1 through 104_N to produce the channels of RF signals. Thecontrol signals ensure that little (e.g., micro to nano amps) leakage orquiescent current is drawn when an RF signal is not being generated. Theleakage and quiescent current can be quite large in high poweramplifiers circuits if not gated. In one embodiment, the RF signals andpower gating signals are turned on and off in 10s of nanoseconds.

The amplified RF signals from the amplifier chains 104_1 through 104_Nare applied to an antenna array 106. The phased array RF signals form asteered composite RF signal to disable a target, typically when it isapproximately 100 meters from the antenna array 106. The steeredcomposite RF signal has Megawatts of radiated power.

System 100 also includes an AC power source 107 for the differentelements of system 100. The AC power source may operate with a powerdistributor 108, which applies power to the power sequencer 105. In oneembodiment, the power distributor 108 converts from AC to DC power.Generally, the conversion from AC to DC can happen either locally ateach amplifier or at the system level.

FIG. 2 illustrates details of certain components in system 100. Centralcomputer 102 includes a processor or central processing unit 200connected to a memory 202. The memory 202 stores instructions executedby processor 200. The instructions include a target classifier 204. Inone embodiment, the target classifier 204 matches the signature of theattributes of the target to a waveform in a waveform look-up table 206.The waveform selector 208 designates a waveform to disable the target.The designated waveform also includes free space parameters to insurethat the steered composite RF signal intercepts the target. The steeredcomposite RF signal is formed by a collection of phase offset RFsignals. The central computer passes RF waveform parameters to the RFsignal generator 103.

In one embodiment, the RF signal generator 103 is implemented as an RFsystem on a Chip Field Programmable Gate array (RFSoC FPGA). The RFSoCFPGA 103 includes a gate array 210 and a direct digital synthesizer 212that creates waveforms of the frequency, pulse width, pulse repetitioninterval and intra-pulse modulation specified by the RF frequencywaveform parameters generated by the central computer 102. The waveformsare passed to a collection of digital-to-analog (DAC) converters 214_1through 214_N. Outputs from the DACs 214_1 through 214_N are optionallyconditioned by filters 216_1 through 216_N. The filters 216_1 through216_N may filter the RF signals to a frequency band of interest. Theoutputs from the RF signal generator 103 are applied to amplifier chains104_1 through 104_N. Each amplifier chain terminates in an antenna ofantenna array 106, such as antennae 220_1 through 220_N.

FIG. 3 is a block diagram of different components of FIGS. 1 and 2,including the RF signal generator 103, power sequencer 105, and anamplifier chain 104_1. The RF signal generator 103 receives a controlsignal from central computer 102 on node 301. A synchronizing clocksignal is received on node 303.

A broadcast signal on node 304, an Ethernet signal in one embodiment, issent to a plurality of power sequencing smart slave units 309. In theone embodiment, the broadcast signal is distributed through a router307. The broadcast signal initiates a calibration mode in smart slavecircuits 509, such that they identify the optimal “on” set point gatevoltage for the power amps 311.

The RF signal generator 103 sends a very fast signal with deterministicdelay, such as a Low Voltage Differential Signal (LVDS) to powersequencer 105. The power sequencer 105 operates as a master powersequencing gating unit that simultaneously controls smart slave devices309. In particular, the power sequencer 105 sends a voltage to the slaveunits 309 and the slave units 309 offset this master voltage with theirindividual voltage offsets that they established in calibration mode, sothat each power amplifier has an optimal gate voltage. Many poweramplifiers have different optimal set gate voltages for “on” operation;the disclosed circuits are configured such that each individual poweramp 311 has its own set point.

The RF signal generator 103 synchronizes using “on” signals applied tothe power sequencers 105. The RF signal generator 103 also applies an RFsignal on node 310, which is propagated through power amps 311. Thepower amp chain may have one or more filters 312. A coupler 313 may beused to allow power levels to be read back to the RF controller 302.More particularly, the feedback includes information on the phase,amplitude, power level and timing of the power amplifiers. This feedbackis taken into account to update timing and control algorithms.

The RF signal is amplified through the power amps 311 and is sent toantenna 314. The output from the different antennae of the antenna array106 form a steered composite RF signal.

FIG. 4 illustrates the RF signal generator 103 applying an “on” signalto the power sequencer 105, which operates as a master power gating andsequencing circuit that controls slave power amplifiers 311. In oneembodiment, this signal is a Low voltage differential signal (LVDS) thatcontrols a switch 403, which causes current from power supply 404 toflow during an “on” state and stops current flow in “off” state. In oneembodiment, the power supply voltage 404 is an offset voltage of about 3volts, which is added to an off voltage V_(OFF) 406 when the switch 403is closed. Since the offset voltage V_(OFF) 406 is about −5 volts andthe power supply voltage 404 is 3 volts, the output voltage on node 409is about −2 volts, which is approximately the gate voltage that turns onGallium Nitride transistors 410. When the switch is open, the outputvoltage on node 409 defaults back to V_(OFF), which is −5 volts in oneembodiment, which is the gate voltage that turns Gallium Nitridetransistors off and reduces leakage current down to about 10 microamps.

Node 411 carries a broadcast signal that initiates the auto-calibrateoperation of the smart slave circuits 309. In one embodiment, each smartslave circuit 309 is implemented with an FPGA configured to determinethe optimal gate voltage set point for turning on a slave amplifier.

Digital to analog converter (DAC) 413 provides an offset voltage thatgets added to the master voltage on node 409. This offset voltage istuned to each individual power amp 410 to provide optimal set point biasvoltage V_(G1) on node 414 and maximum power out from the power amp 410.It also enables optimum voltage in the “off” state and minimizes leakagecurrent. The master-slave architecture facilitates fine grained voltageoffsets, which is critical to the operation of many transistors, whichmay be sensitive to gate voltage offsets at the millivolt level. Thedisclosed technology maximizes voltage offset resolution.

The smart slave 309 controls a plurality of DACs 413 and storesdifferent optimum set points for both the on and off states for eachpower amp. In the auto-calibration mode, the current sensor 415 is usedto feed back a current reading to the smart slave 309. This voltageoffset on node 413 is tuned very slightly, by the millivolt in oneembodiment, until the current sensed from 415 reaches an optimum currentvalue, as per the data sheets for the power amps 410. This voltageoffset is stored. This process is repeated to minimize the current in“off” state. The current can also be sensed during active operation todetermine the viability of the power amp. If the current starts todegrade or change or significantly decrease, this can indicate that theamplifier is damaged and needs to be replaced, or can indicate that thetemperature is out of range for optimal operation.

The capacitor 416 can be tuned to change the rise and fall time for thegate bias signal on node 414. In some embodiments, capacitor 416 is realtime programmable by the smart control FPGA 309, such as by a series ofswitches, to include more or less capacitance in the feedback path 416.This is important because different power amps 410 each have a differentgate capacitance. Capacitor 416 is tuned based on the gate capacitancefor optimal operation. Tuning capacitor 416 affects how fast or slow therise time is on the gate voltage at node 414, this effects speed andefficiency of the power gating. Changing the charge on capacitor 416 canalso change the amount of time the power amp rings or oscillates. Inother embodiments, capacitor 416 is configured to tune the rise and falltime for very fast operation.

FIG. 5 illustrates waveforms that may be used in conjunction with thecircuitry of FIG. 4. The supply voltage 501 (V_(SUPPLY) in FIG. 4) tothe power amp (410 in FIG. 4) is turned on first. Alternately, it may beleft on all the time. The gate voltage waveform 502 is applied to node414 of FIG. 4. Then, the RF signal from RF signal generator 103 isapplied to node 414. This example is for a 65 Volt Gallium Nitride (GaN)solid state power amplifier, but the principle may generally apply toany solid state power amplifier. The drain voltage 501 toggles from 0Volts to 65 Volts. Then, the source current is tuned from −5 Volts to −2Volts, where it is considered “open” and the transistor is “on” so thata quiescent current starts to flow. Finally, the RF input signal 503 isapplied and the transistor draws active power once the RF power is on,in some embodiments up to 30 amps of current create 1,500 watts of powerout of the transistor 410.

The RF signal 503 is sent out as a short pulse, for example, as short as10 ns or as long as milliseconds. The length of the pulse depends on thetype of target. After the RF pulse is complete, the source voltage ispinched off back down to −6 Volts, and then shortly after the drainvoltage is tuned from 65 Volts down to 0 Volts and the transistor is offand therefore draws minimal current.

FIG. 6 illustrates a timing diagram showing a non-linear pulse train 601with uneven pulses. The pulse train 801 is sent through power amp 410,where the RF and voltage bias is turned on and off very quickly (e.g.,10s of nanoseconds). In one embodiment, the pulses are in an arbitrarypattern at a frequency of 1 GHz.

FIG. 7 illustrates a system 700 corresponding the system 100 of FIG. 1.However, in this embodiment, the antenna array 106 transmits its RFpower signal to a reflector 700. For example, 16 antennae operating atthe L-band frequency with half-wavelength spacing may transmit into a 3meter reflector dish. The reflector dish may have a subreflector. Amechanical gimbal 702 may control the position of the reflector 700 inresponse to control signals from central computer 102.

The 3 meter reflector dish provides 28.1 dBi, or 645× linearmagnification of the energy. In one embodiment, the reflector dish isfed by a 16 element phased array antenna in a 4×4 array. At a 1% dutycycle and 70% power efficiency, the power system only requires 550 wattsof DC power output, enabling a small power supply.

The foregoing description, for purposes of explanation, used specificnomenclature to provide a thorough understanding of the invention.However, it will be apparent to one skilled in the art that specificdetails are not required in order to practice the invention. Thus, theforegoing descriptions of specific embodiments of the invention arepresented for purposes of illustration and description. They are notintended to be exhaustive or to limit the invention to the precise formsdisclosed; obviously, many modifications and variations are possible inview of the above teachings. The embodiments were chosen and describedin order to best explain the principles of the invention and itspractical applications, they thereby enable others skilled in the art tobest utilize the invention and various embodiments with variousmodifications as are suited to the particular use contemplated. It isintended that the following claims and their equivalents define thescope of the invention.

1. An apparatus, comprising: a Radio Frequency (RF) signal generator toproduce RF signals phase shifted relative to one another in accordancewith RF frequency waveform parameters; and amplifier chains to processthe RF signals to produce channels of amplified RF signals, wherein eachamplifier chain has amplifiers and wherein at least one amplifier has atunable gate voltage synchronized with the RF signals.
 2. The apparatusof claim 1 wherein each amplifier chain has a serial sequence of solidstate amplifiers each of which has a tunable gate voltage.
 3. Theapparatus of claim 1 wherein the tunable gate voltage is an amplifier onset point that is derived from an automatic calibration operation. 4.The apparatus of claim 1 wherein the at least one amplifier has acapacitance that is tuned to an on set point for the at least oneamplifier.
 5. The apparatus of claim 1 wherein the at least oneamplifier has a gate voltage tuned based on sensor feedback from the atleast one amplifier.
 6. The apparatus of claim 1 wherein an offsetvoltage of a plurality of gate voltage slave circuits is tuned andcontrolled by a central master power gating circuit.
 7. The apparatus ofclaim 1 further comprising a central computer to produce the RadioFrequency (RF) waveform parameters.
 8. The apparatus of claim 7 furthercomprising an antenna array to broadcast the channels of amplified RFsignals as a steered composite RF signal with Megawatts of radiatedpower.
 9. The apparatus of claim 8 wherein the steered composite RFsignal is pulsed for less than 1 millisecond.
 10. The apparatus of claim8 wherein the steered composite RF signal has a frequency ofapproximately 1 GHz.
 11. The apparatus of claim 7 wherein the centralcomputer includes a processor and a memory storing a target classifierwith instructions executed by the processor to classify a target basedupon flight attributes of the target.
 12. The apparatus of claim 11wherein the central computer includes a waveform selector stored in thememory, the waveform selector configured to select the RF waveformparameters from a waveform look-up table.
 13. The apparatus of claim 1wherein the RF signal generator is an RF system on a Chip FieldProgrammable Gate Array.
 14. The apparatus of claim 1 wherein the RFsignal generator produces digital RF signals that are applied todigital-to-analog converters.
 15. The apparatus of claim 1 furthercomprising a power sequencer controlled by the RF signal generator. 16.The apparatus of claim 15 wherein the power sequencer is configured as amaster power sequencing gating unit.
 17. The apparatus of claim 16further comprising smart slave circuits controlled by the powersequencer, wherein the smart slave circuits coordinate an automaticcalibration operation.
 18. The apparatus of claim 1 further comprising areflector dish to process the channels of amplified RF signals.
 19. Theapparatus of claim 18 further comprising a mechanical gimbal to orientthe position of the reflector dish.
 20. The apparatus of claim 1 incombination with a target detector.